This invention relates to audio circuitry and more particularly to circuitry for reducing electrical noise which produces undesirable and annoying ticks and pops, etc. in reproduced sound.
Audio signals which are broadcast or recorded as for example on a video disc are transmitted/recorded in the form of a modulated carrier. During transmission or recording/recovery the signal will typically have imposed thereon electrical noise in the form of impulses or suffer periods where the signal is lost entirely, e.g., "drop outs". The impulses or dropouts generate undesirable disturbances in the reproduced sound signals.
It is known to reduce the affects of electrical noise by limiting the signal bandwidth and/or limiting signal amplitude to a predetermined threshold. The affects of signal dropouts have been reduced by detecting the occurrence of a drop out and holding the baseband audio signal constant at a level equal to the signal level occurring immediately prior to the dropout for the duration of the dropout, see for example U.S. Pat. No. 4,221,930 entitled, "FM Defect Compensation Apparatus" issued to Y. Okuno.
Typically, noise reduction in audio systems is performed on the baseband signal. It has been found, however, that what start out as very narrow noise pulses (e.g., 2 .mu.sec) in the RF section of a receiver/player may ultimately be manifested as a much longer (e.g., 20 .mu.sec) disturbance in the reproduced sound. This occurs due to fast noise pulses causing ringing in for example the IF filter section. It should be appreciated that an impulse noise disturbance in the signal is expanded to equal the duration of such ringing. Thus, it is advantageous to perform noise reduction at the RF section of the receiver/recovery apparatus before such noise expansion occurs.
K. Amazawa et al. in U.S. Pat. No. 4,178,552 describe a "Noise Eliminating Circuit" which comprises an oscillator to track and generate a signal having a frequency equal to the modulated carrier frequency, which generated signal is substituted for the carrier when the carrier contains noise. This apparatus, however, does not have facility to adapt to changing carrier amplitudes, thus switching between the received carrier and the generated signal may itself create disturbances in the reproduced sound.
The present invention substitutes delayed signal for noisy real time signal. It is noted that a similar technique has been utilized to eliminate noise/dropouts in recorded video signals for many years, see for example U.S. Pat. No. 3,463,874. These systems however rely on the fact that video signals are highly redundant line-to-line so that substitution of signal from an adjacent line will correlate closely with the substituted signal. Audio signals on the other hand generally are not correlated with any predictable regularity. The present inventor realized that substitution of a short segment, e.g., 1-2 .mu.sec. of the RF audio signal with an adjacent segment of RF signal can be considered analogous to sampling and holding the signal for a brief period. Therefore, as long as the substitution periods are relatively short compared to the minimum sound signal wavelength, correlation of the substitution signal with the substituted signal is inconsequential. For a given noise impulse, the substitution (i.e., holding) period required in the RF section is significantly less than the sample and holding period required at baseband, thus the signal substitution system in the RF stages generally enhances ultimate system response.